It is necessary to supply a gate drive current to the gate electrode of a thyristor in order to turn the thyristor on. However, it is not necessary to maintain current flow to the control electrode after the thyristor is turned on.
A problem arises in the utilization of power thyristors in that the gate drive power supply must be sized to provide the current necessary to initiate conduction of the thyristor, and in former designs the gate drive power supply must continue delivering current to the control electrode of the thyristor during conduction of the thyristor. The continuing supply of current to the control electrode of the thyristor requires that the gate drive power supply be sized larger than necessary. Also, supplying continuous gate drive current is an unnecessary waste of electrical current, particularly since it tends to simply heat the power supply and accomplish no useful function. A further problem is to find a way to limit current flow to a thyristor gate during full conduction operation of the motor.